Travelling field synchronous AC motor

ABSTRACT

The invention relates to a synchronous AC electric motor with concentrated windings ( 2 ), whereby one primary side pole is comprised of a series of toothed-modules ( 4 ) with each toothed-module ( 4 ) being connected in the correct electrical phase sequence to a corresponding phase ( 11 ) of the motor electrical supply ( 5 ). In order to provide a highly optimized concentrated winding motor design applicable equally to either linear or rotating machines, which retains the performance benefits of existing concentrated-winding motors it is proposed that neighboring modules ( 4 ) are wound so as to have alternating polarities, whereby the number of toothed-modules ( 4 ) comprising a primary side pole is exactly equal to twice the number of motor phases ( 5 ), and that the motor is designed to operate using an air-gap flux component which is harmonic of the fundamental phase current frequency.

BACKGROUND OF THE INVENTION

The invention relates to a method for reducing torque ripple in ACbrush-less motors with concentrated windings. AC synchronous travellingor rotating field motors with concentrated primary side windings i.e.windings in which individual coils, wound onto single primary sidetoothed-modules are connected together to form one phase of the motorwinding, have significant performance benefits when compared to standardrotating AC field motors with overlapping or so called distributed orsine-weighted windings.

These benefits have been well understood for several years and includereduced power loss, increased torque density, simplified and more highlyautomated manufacturing processes and lower tooling costs. All of theseadvantages spring from the fact that the concentrated winding occupiesless physical space than a distributed winding.

A distributed winding must traverse the motor primary side, and must bewound such that the coils of one winding overlap with those of a secondwinding. The difficulties in physically fitting such windings into theavailable space on the primary side result in reduced slot fill factori.e. less wire in each of the primary side slots, and large lengths ofwire running between the primary side slots at the ends of the motor.These so called end-windings do not contribute to torque production, butdo increase the winding resistance and thus the heat losses which areproportional to the resistance, and thus reduce motor efficiency.

Concentrated winding motors on the other hand, are usually formed bywinding coils onto individually produced primary side toothed-modules,which are themselves built up from multiple laminations. The woundmodules are subsequently welded or otherwise joined together to form theprimary side of the motor.

SUMMARY OF THE INVENTION

This kind of motor is described in U.S. Pat. No. 5,729,072 which showshow the physical construction of the winding enables the slot-fillfactor to approach the theoretical maximum, while reducing the length ofthe end winding to an absolute minimum. This combination of desirablecharacteristics tends to produce a motor which, for a given torque, hasa very low volume and is highly suitable for applications were space isat a minimum, such as robotic or materials handling applications.

The concentrated-wound type of motor has however a number of undesirablecharacteristics, both electromagnetic and physical, which can limit itsuse in servo applications. The concentrated nature of the primary sidewindings results in a magnetic system which tends to produce high levelsof harmonics in the air-gap flux which, when coupled with correspondingharmonics in the magnetising flux circuit, create unwanted variations inthe motor output torque or force. This torque ripple must be compensatedfor in the servo control system if the resulting motor speed is not todisplay an unacceptable level of ripple.

In order to avoid this, special measures must be taken in the design ofconcentrated winding motors in order to reduce the levels oftorque/force ripple present in the output of the motor, which wouldotherwise reduce the absolute performance of the drive system. Thesemeasures include skewing the magnets on the secondary side orskew-magnetising the secondary side itself or zoning the winding whichinvolves distributing the winding across several slots within onetoothed-module. Even after such measures are taken, the motor's outputtorque can still contain a significant element of ripple.

Furthermore, the requirement that the toothed-segments be woundindividually and subsequently joined together, place large demands onthe accuracy of the manufacturing process as great care must be taken injoining the segments together in order to preserve the magneticproperties of the motor primary side itself.

Additionally, care must be taken in the magnetic circuit design toreduce the reluctance and cogging torque components which are due to thevariation in secondary side position and the primary side inductance,which ultimately also appear as non-linearities in the motor's outputtorque or force.

These optimisation processes which are required to overcome the inherentdisadvantages of the design, all tend to increase the manufacturing costof the motor and reduce its ultimate efficiency.

It is the object of the present invention to provide a highly optimisedconcentrated winding motor design applicable equally to either linear orrotating machines, which retains the previously discussed performancebenefits of existing concentrated-winding motors, such as high torqueand reduced losses while further improving the manufacturability of themotor, reducing its production costs, increasing its torque producingefficiency and simultaneously improving its performance by reducingnon-linearities in the output torque which reduces the demands on themotor servo control system.

The current invention achieves these aims in that a concentrated-windingrotating-field AC synchronous motor with a number P current carryingmotor phases, P₁ to P_(N), is constructed such that each electromagneticpole of the primary side is comprised of a sequence of N toothed-moduleseach wound with a coil, whereby the coil of each successive module isconnected to the next electrical phase, following in the correctelectrical sequence. The toothed-modules are subsequently wound so as tohave alternating polarities, and the number of modules N is equal totwice the number of motor phases i.e. N=2×P.

The winding pattern and primary side geometry which is the object of thecurrent invention, results in a travelling electromagnetic field in themotor air-gap which, when decomposed into its components by means of amathematical method such as the Fourier Transform, can be demonstratedto be composed of a number of space harmonics with predictablefrequencies and magnitudes, one of which is chosen as the base frequencyfor driving the motor.

The torque producing space harmonics are multiples of the fundamentalphase current frequency which is itself also present in the air-gapmagnetic field, however, the motor secondary side is designed so as toproduce a magnetising field which has no component at the fundamentalphase current frequency. This is achieved by controlling the spatialdistribution of the magnets on the secondary side magnet carrier. Thephysical positions of the magnets with respect to each other and to themotor primary side is a critical factor in determining thecharacteristics of the motor. As motor torque is due to the interactionof an electromagnetic field component with a corresponding magnetisingfield component, there is no output torque at the fundamental frequency.

A similar argument can be applied to other higher frequency componentsof the air-gap electromagnetic field, whereby due to the choice ofprimary side geometry and winding pattern, the components either do notexist or have been deliberately reduced in magnitude, so as to eliminateor reduce the corresponding torque components.

The resulting motor has an output torque which contains significantlyreduced levels of ripple torque, i.e. variations of torque with positionor current, in comparison to other standard concentrated winding motors.

This results in a synchronous motor which can be more easily andaccurately regulated when used as an actuator in a servo control system,for example using a standard PID velocity control system, a givenmaximum allowable velocity ripple could be achieved with a reducedcontroller gain factor, thus improving the gain margin and the systemstability.

A main benefit of the invention is the significant reduction in thevariation in motor output torque with rotor or secondary side position,achieved by among other things, the use of a motor operating frequencywhich is a harmonic of the fundamental phase current frequency. In orderto achieve this, an air-gap flux is generated with harmonic componentswhich are significant in amplitude in comparison to the fundamentalfrequency component. The invention thus describes a motor primary sidewinding pattern whereby the coils on the toothed-modules are wound so asto form a sequence of alternating polarities. I.e. the coils are woundalternatingly in a clockwise and anticlockwise sense.

In other words, the coil or winding on each toothed-module in thesequence is wound and connected to one of the phases of the motor ACelectrical supply such that, if it were possible to view the primaryside from the secondary or air-gap side at a point in time when allmotor phase currents were positive and flowing into the motor (which isin reality an impossible situation but helpful for understanding), andall coils were energised, the polarity of the magnetic field in the airgap created by the toothed modules would alternate . . .-North-South-North-South- . . . . This is denoted, for motor phases P1to Pn by the following electrical winding pattern:

-   P1 −P1, −P2 P2, P3 −P3 . . . Pn −Pn, −P1 P1, P2 −P2, −P3 P3, . . .    −Pn Pn.

Any electromagnetically equivalent winding pattern i.e. any windingpattern which by virtue of a symmetrical invariance with the pattern ofthis invention, produces an equivalent electromagnetic field, may alsobe used to produce the same result.

In the present invention, the number of and thus physical size of thesecondary side magnets, for a given secondary side diameter and a givenprimary side geometry, is related to the frequency of the torqueproducing component of the electromagnetic field. The use of a frequencycomponent which is a multiple of the fundamental phase currentfrequency, dictates that the magnet size be proportionately reduced incomparison with a similar standard AC field motor which makes use of thefundamental frequency, as the wavelength of the higher frequencycomponent is proportionately shorter than that of the fundamentalcomponent. The individual magnets on the secondary side are thus smallin comparison to the length of a primary side magnetic pole. This allowsthe air gap flux to be optimised by adjusting the spacing betweenindividual magnets on the secondary side, such that the variations inair-gap inductance around the motor are minimised. Thus, without theaddition of structures such as pole shoes, as shown in U.S. Pat. No.5,729,072, the reluctance and cogging torque generated by the variationsin inductance with angle in the magnetic circuit of the motor can beminimised.

The invention describes a synchronous motor with primary side slots withpractically parallel sides with no pole-shoe, which thus benefit fromhaving wide apertures facing the air-gap. This mechanical structureallows individual windings to be easily inserted in situ onto thetoothed-modules on the motor primary side, thus allowing theconstruction of a high slot-fill factor motor, while at the same timeremoving the requirement for a time consuming winding process.

It is an advantage of the present invention that the primary side of aconcentrated winding motor is produced from a stack of laminations of asuitable geometry, each of which is formed as a single piece, byprocesses such as stamping or laser cutting which are commonly used inthe production of standard AC motors. Each lamination contains all thetoothed modules required to construct a complete primary side, so forexample each of the single laminations for a cylindrical rotationalmotor are circular in form. This results in an inherently rigid andstable motor structure which does not require further processing stepssuch as welding or gluing common to the production of concentratedwinding motors as, for example, described in U.S. Pat. No. 5,729,072.

Due to the elimination of the pole-shoe, it is physically possible toinsert, in situ, a pre-wound coil with a geometry corresponding to thephysical geometry of a motor primary side tooth, directly onto theprimary side tooth in a completed primary side. Thus in a further novelfeature, the invention allows the flexibility of manufacturing a highslot-fill factor, concentrated winding motor primary side, from a stackof single laminations using processes common to the production ofstandard rotational or linear AC motor primary sides.

A further embodiment of the invention encompasses a linear ACsynchronous motor with multiple primary side poles, whereby in order toallow some flexibility in the sizing of the toothed-modules for a givenmotor primary side length, the winding pattern can be non-symmetrical orincomplete at one end of the motor. Thus, for example, in a linear ACsynchronous motor with four primary side poles and phases P1 to Pn thefollowing winding pattern would be repeated 3 times, once for each pole:

-   P1 −P1, −P2 P2, P3 −P3 . . . Pn −Pn, −P1 P1, P2 −P2, −P3 P3, . . .    −Pn Pn,    and the final section would have the incomplete pattern:-   P1 −P1, −P2 P2, P3 −P3, . . . Pn −Pn

In a further embodiment the ratio of secondary side pole length τp (6)to stator slot spacing τm (7) is such that

${\frac{\tau\; p}{tn} = {\frac{m^{\prime}}{m^{\prime} + 1}q^{\prime}}},$whereby for a symmetrical winding the number of virtual motor phases m′is equal to the number of real motor phases (5) m, and for anasymmetrical winding is equal to 2×m, and whereby the factor q′ is equalto half the length of one toothed-module (4), ie,

${q^{\prime} = \frac{z}{2}},$when the module is wound with a free slot (13), and otherwise

${q^{\prime} = {q = \frac{N}{2 \cdot p \cdot m}}},$a whole number, where p represents the number of motor primary sidepoles (3), and N represents the number of primary side slots (13).

In a further preferred embodiment of the invention, the primary side ofa concentrated winding motor is designed to maximise the output torque,in that the ratio of secondary side pole length to primary sideslot-width to is 6:5. This produces an electromagnetic air-gap fluxwhich allows the secondary side to be optimised so as to eliminate the7^(th) harmonic component in the output torque and minimise the 11^(th)and 13^(th) harmonic components. This results in a motor which isoptimised for maximum output torque while maintaining acceptable torqueripple and retaining a magnet size which is easy to handle in themanufacturing process.

In a further preferred embodiment of the invention, the primary side ofa concentrated winding motor is optimised such that the ratio ofsecondary side pole length to primary side slot-width is 6:7. Thisproduces an electromagnetic air-gap flux which allows the 7^(th)harmonic to be used as the base frequency. The advantage of thisgeometry is a significantly reduced torque ripple due to the 5^(th)harmonic component of the electromagnetic flux having no counterpart inthe magnetising field, which is itself designed to operate with the7^(th) harmonic.

The required magnet size decreases with the increase in motor basefrequency, thus for a given motor size it becomes difficult to handlethe magnets once secondary side pole length to primary side slot-widthratios increase beyond a given level. Ratios of 6:5 and 6:7 have beenfound to be best suited to the production of small and medium sizedservo motors, however for larger motors, ratios of 6:11, 6:13 or higherbecome practical and allow further optimisation of the magnetic circuitgeometry.

In another preferred embodiment of the invention the number of motorphases is equal to 3, resulting in a concentrated winding synchronous ACmotor which is compatible with all standard AC motors, and thus allstandard converters, inverters and mains supply topologies. This allowsthe motor to be operated using readily available 3 phase power controlsystems.

The primary side winding is thus connected such that in a standard3-phase motor with motor phases a, b, and c, adjacent coils are woundsuch that one complete magnetic pole of the primary side, whichcomprises of a sequence of six toothed-modules, has a winding pattern: a−a, −b b, c −c, a a, b −b, −c c where a minus sign indicates the side ofthe coil at which a positive motor current, i.e. a current flowing intothe motor phase from the supply, would flow out of the respective coil.The resulting space vector magnetic field obtained from such a windingpattern may also be achieved using winding patterns which aregeometrically equivalent, arrived at by employing techniques such as,for example, zoning.

The magnetising field of a synchronous AC motor is, in general, producedby multiple permanent magnets arranged on the secondary side, althoughin larger machines it may be beneficial to produce the secondary sidefield electromagnetically, the magnets being located physically on thesecondary side radially or diametrically opposed to the primary side.There are a number of mechanical variations possible on the secondaryside such as the use of surface magnets or buried magnets, the currentembodiment of the invention uses surface-mounted magnets which have afixed correspondence to the primary side's toothed-modules.

The current invention can be more clearly understood by reference to thefollowing diagrams whereby FIGS. 1 and 2 together define a preferredembodiment.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 Shows a longitudinal schematic cross-section of the primary sideand corresponding permanent magnet secondary side of a 3-phase motorwhich describes a first embodiment of this invention.

FIG. 2 Shows a sectioned plan-view of the primary side of a 3-phaselinear synchronous AC motor corresponding to the claims of the currentinvention.

FIG. 3 Shows an example of ‘zoning’

FIG. 4 Shows a primary side tooth with pole-shoe

FIG. 5 Shows graphical comparisons of torque versus electrical angle fora standard concentrated winding motor and the current invention.

FIG. 6 Shows a section of three-dimensional drawing of a motor primaryside without coils constructed from laminations

FIG. 7 Shows a pre-wound copper coil insertable into the slots of themotor primary side.

DESCRIPTION OF THE PREFERED EMBODIMENTS

FIG. 1 shows a front view schematic of a multiphase synchronous AC motorin cross section, whereby the stator or primary side (1) is constructedof a sequence of toothed-modules (4) which are themselves composed ofone or more teeth (9). The schematic of FIG. 1 can be thought of asshowing a cross-section of the primary side (1) of a cylindricalmachine, which has first been ‘sliced open’ down its length and rolledout flat. This structure can be used to represent the functioning of alinear or a cylindrical machine.

The motor primary (1) and secondary (16) sides are made up of a numberof thin ferromagnetic steel sheets known as laminations which have across-section as shown in FIG. 1, these laminations are stacked togetherto form the motor primary (1) and secondary (16) sides. The motor mayhave a further mechanical structure or housing used to hold the stackedlaminations.

Each of the primary side (1) laminations displays a repeating ‘T’ shapedpattern along its length, the basic unit of which is called atoothed-module or simply module (4). The modules are themselvescomprised of one or more substantially ‘T’ shaped teeth (9), with thehorizontal section of the ‘T’ being common to each of the one or moreteeth (9) in the module (4), and the vertical section representing onesingle tooth (9). This results in the modules having either a simple ‘T’shape, or a comb-like structure.

The gaps between the teeth (9) are known as slots (13), and each slot(13) contains one half of one concentrated winding (2). Each winding ismade up of one or more ‘turns’ of wire which are wrapped around, orpre-formed and inserted around, a ferromagnetic tooth (9), to form whatis, in effect, an electromagnet.

Each coil (2) is designed to carry a current (11) which creates amagnetic field whose shape and strength is determined by the geometry ofthe ferromagnetic structures in the motor.

There are two sources of magnetic energy in the motor, permanent magnets(14) and current carrying coils (2). The field created by the permanentmagnets (14) is referred to as the magnetising field, and that producedby the phase currents (5) flowing through the coils (2), as theelectromagnetic field. The orientation of the magnetic fields i.e. N-Sor S-N is referred to as the polarity of the field.

The coil (2) on each toothed-module (4) is wound such that the polarityof the resulting electromagnetic field is the same regardless of whetherthe module (4) has a simple ‘T’ shape, and thus a simple concentratedwinding (FIG. 1), or a comb-like structure and thus a zoned winding(FIG. 3), where within one module the winding is distributed over anumber of teeth. That is to say, a positive current flowing into eitherof these windings (2) would, in both cases, produce a magnetic flux inthe associated ferromagnetic tooth material with the same polarity orspatial orientation. Moreover, the coils (2) on the toothed-modules (4)are wound such that when a positive current is flowing into all coilsi.e. a flow of current into the motor, consecutive or neighbouringmodules (4) always have opposite polarities. The magnetic flux, which isdirectly proportional to the magnitude and phase of the current flowingin the windings (2), flows in a magnetic circuit comprising principallyof the primary side (1), the secondary side (16) and the air-gap betweenthem (10). This flow of flux from primary side (1) to secondary side(16) and back, creates the torque-producing magnetic field or flux inthe air-gap (10). The coil (2) of each individual module (4) isconnected to a single phase (11) of the motor supply, wherebyconsecutive modules (4) are connected to consecutive phases of the motorsupply. All phases (5) carry essentially sinusoidal currents of equalmagnitudes but with varying electrical angles, i.e., each phase currentin the sequence is offset from the previous phase current (5) by a fixedangle equal to

$\frac{2\pi}{P}$radians, where P is the number of motor phases.

The magnetic fields produced by neighbouring windings (2) interact toproduce a resultant air-gap flux which is dependant upon the geometry ofthe ferromagnetic components, the geometry of the windings (2), and themagnitude and phase of the winding currents (5).

This resultant air-gap flux is a vector quantity which varies both intime and space, whose frequency components can be analysed bytransformation from the time domain into the frequency domain. This canbe achieved by using a suitable mathematical transformation such as theFoiurier transform. The time varying flux waveform can thus be shown tobe made up of a sum of individual waveforms at frequencies which areinteger multiples of the supply current (11) or fundamental frequency,each of which has a magnitude which is directly linked to the geometryand winding pattern (15) of the primary side (1), and each of whichpropagates or travels around or along the air gap (10).

The motor can be optimised to use any of the frequency components of theair-gap flux as the torque producing component, this frequency thenbeing referred to as the motor base frequency.

The performance of the motor is also closely tied to the relationshipbetween the geometry of the secondary side (16), and the geometry of theprimary side (1), a key measure of which is the ratio between the lengthof, or circumferencial angle subsumed by one primary side tooth-module(7), and the corresponding length of, or circumferencial angle subsumedby, one secondary side pole (6). A secondary side pole length orcircumferencial angle (6) being defined as the total length of the motorsecondary side active element, or 2·π radians, divided by the number ofmagnetic poles on the secondary side (16). One secondary side (16)magnetic pole (6) comprises, in effect, one magnet and one half of thegap between it and its neighbouring magnets on either side. A primaryside pole is defined as the number of modules required to form acomplete pattern (15), e.g. a 3-phase machine with winding pattern (15):

-   a −a, −b b, c −c, −a a, b −b, −c c, would require 6 modules. A ‘P’    phase machine would thus have 2×P modules (4) per primary side pole.

The secondary side (16) is constructed as shown in FIG. 1 with multiplepermanent magnets (14) being fixed onto a laminated ferromagneticcarrier whereby a small gap of defined dimensions (20) is left betweenthe magnets (14), and subsequent magnets have alternating polarities.The magnets (14) are orientated such that they are, as far as possible,parallel to the toothed-modules (4) on the primary side (1).

The distance between the secondary side (16) and primary side (1) ismechanically fixed to produce an air-gap (10) of, ideally, constantdimensions, the absolute size of which is dependant upon the type andperformance of the motor being built.

On the primary side (1) of a ‘P’ phase machine, the alternating polarityof the concentrated windings (2) produces the geometric pattern

-   P1 −P1, −P2 P2, P3 −P3 . . . Pn −Pn, −P1 P1, P2 −P2, −P3 P3, . . .    −Pn Pn, which in a 3 phase machine with phases a, b, and c, such as    that shown in FIG. 2, results in the winding pattern (15):-   a −a, −b b, c −c, −a a, b −b, −c c, where the individual motor    phases (11) are connected to the windings (2) as shown, and the    windings (2) are connected to each other at a ‘star’ point (17).

The phase currents (5) are sinusoidal in form and have a fundamentalfrequency component cos(ωt−α), i.e. each phase (11) rotates at avelocity ω radians per second, and each phase (11) precedes the next byan electrical angle of 120° or

$\frac{2\pi}{3}$radians, thus:

${{U \equiv a} = {\cos\left( {\omega\; t} \right)}},{{V \equiv b} = {\cos\left( {{\omega\; t} - \frac{2\pi}{3}} \right)}},{{W \equiv c} = {\cos\left( {{\omega\; t} - \frac{4\pi}{3}} \right)}}$

Where U, V and W are common descriptors for the phases (5) of a standardthree phase AC motor.

The phase currents (5) flowing in the windings (2) generate a magneticflux in the motor ferromagnetic primary side (1), the motor geometrybeing such that the flux path is practically radial or normal and acrossthe air-gap (10) between the primary side (1) and secondary side (16).The sum of the air gap flux components in space and time is itself asinusoidal quantity which propagates around, or in the case of a linearmotor along, the air-gap (10). This travelling wave interacts with themotor magnetising flux produced by the permanent magnets (14) to producea torque or force output.

The flow of current in the windings (2), which are ordered so as to havea winding pattern (15), results in an air-gap flux which is a complextime domain waveform, which can be represented in the frequency domainby a series of sinusoidal components at the fundamental frequency andinteger multiples thereof.

The magnitude of the fundamental, or lowest frequency, component of theair gap flux produced by the winding pattern (15), can be shown to besignificantly attenuated in comparison to that of the 5th and 7thharmonics. This feature of the invention allows a harmonic of thefundamental frequency to be used effectively as the base frequency fordriving the motor, and not the fundamental frequency itself as is commonpractice.

Furthermore, as the air gap magnetising flux produced by the permanentmagnets (14) is designed so as to have no component at the fundamentalfrequency ‘ω’ of the rotating or travelling electromagnetic field, thereis no corresponding torque or force component in the motor output. Thetorque producing components of the air-gap flux space harmonicsgenerated by the primary side phase currents (5), are those componentswhich interact with equivalent components of the magnetising fieldcreated by the arrangement of permanent magnets (14). For example, in amotor with a geometry as shown in FIG. 2 whereby the pole (6) to slot(7) ratio is 6:5, the main torque producing components are the 5^(th),7^(th) and 13^(th) harmonics.

It is possible to ‘tune’ the torque producing components of the air-gapflux, by modifying the geometry of the toothed-modules (4) usingtechniques such as zoning, FIG. 3, without in any way modifying thebasic winding pattern (15). By such means it is possible to accentuatethe desired torque producing components of the air gap flux, whileattenuating or eliminating other components which would otherwisecontribute only to ripple in the output torque (19).

A further unwanted torque component is produced by the changes inmagnetic circuit geometry which occur as the secondary side (16) movesrelative to the primary side (1). The flux, which is produced by thepermanent magnets (14), is forced to flow in a magnetic circuit withvarying levels of reluctance. The attractive and repulsive reluctanceforces or torques thus produced tend, because of the unavoidablesymmetry of a motor's geometry, to sum together around or along themachine. Suitable placement of the secondary side magnets (14) withrespect to each other, and to the primary side teeth (9), can reducethis effect, however, in standard designs it is necessary to include apole shoe structure (12) i.e. to widen the tooth towards the air—gap(10) end, in order to minimise changes in reluctance with magnet (14)position. The use of a harmonic of the fundamental supply currentfrequency as the motor base frequency, allows the use of magnets (14)which are small in comparison with a primary side pole. This allows thesecondary side (16) to be optimised by the use of suitable magnetspacing (20), and pole (6) to slot (7) ratios, so that even without theuse of pole-shoes (12), the sum of reluctance torque around or along themachine always tends to zero.

An immediate benefit of the lack of a pole-shoe (12), is the possibilityof inserting a pre-wound coil (2) directly onto a tooth (9) in acompleted primary side (1). This removes the need for producing single,wound, toothed-modules (4), which then must be joined together in afurther manufacturing process such as welding or gluing.

As stated, optimal designs for such a motor have been mathematicallyderived and tested, and have been found to have ratios of 6:5, 6:7,6:11, 6:13 and higher. The higher the ratio, the higher the basefrequency, and thus the smaller the relative size of the magnetrequired, i.e. the magnet size is directly proportional to the motorsize. At some point a secondary side pole width (6) becomes so smallthat the magnets can no longer be easily handled in the manufacturingprocess. Ratios of 6:11, 6:13 and higher result in magnet (14) sizeswhich would be applicable to the construction of physically largemotors, for example rotational AC synchronous permanent magnet motorswith a diameter of greater than 20 cm, where the magnets (14) wouldstill be large enough to handle easily.

The benefits of the invention can be clearly seen when a comparison ismade between the output torque of a motor with a standard concentratedwinding construction, and that of a motor with a construction asembodied-in the present invention, the results of which can be seen inFIG. (5). The level of torque/force ripple i.e. deviation about a meantorque level (19), produced by a motor constructed following theteachings of the present invention, measured as a percentage of themotor average torque/force, can be seen to be significantly lower thanthat of a conventional concentrated winding design (18). The standardmotor has a somewhat higher maximum torque, but this is in reality notuseable, because ultimately the level of torque ripple dictates themotor and servo system performance.

The significant reduction in torque ripple which is possible in a motordesign incorporating the concepts of the current invention, results inan increase in useable motor torque, and thus an increase in the torqueproducing efficiency of the motor itself as measured in Nm.A⁻¹ or N.A⁻¹.

It is thus clear that the advantages of the present invention areobtained by creating a synchronous AC motor with concentrated windings(2) on the primary side (1), having a geometry and winding pattern (15)which results in a magnetic space wave whose main component is aharmonic of the supply current frequency, which interacts with acorresponding component of the magnetising field on the secondary side(16) to produce a substantially ripple free torque.

LIST OF FIGURES

-   1. ‘P’ phase motor stator-   2. Concentrated winding-   3. Standard concentrated winding-   4. Toothed module-   5. Motor supply phases-   6. Secondary side pole length-   7. Slot spacing-   8. Tooth edge-   9. Tooth-   10. Air gap-   11. Phase current-   12. Pole shoe-   13. Slot-   14. Permanent magnets-   15. Winding pattern-   16. Secondary side-   17. Star connection point-   18. Motor output torque curve-   19. Motor output torque curve-   20. Inter-magnet gap

1. A linear traveling field synchronous AC electric motor with a primaryside (1) comprising concentrated windings (2); a series oftoothed-modules (4) each wound with a coil (2), whereas eachtoothed-module (4) can comprise a single or a plurality of teeth (9),the coil of each successive toothed-module (4) being connected in thecorrect electrical phase sequence to a corresponding phase (11) of themotor electric supply (5), whereby each toothed-module (4) shows acontrarily winded winding and a contrarily applied current compared toits neighboring toothed-module (4), and the number of toothed-modules(4) is exactly equal to twice the number of motor phases (5),characterized in that a secondary side (16) arrangement is comprised ofa plurality of magnets (14) which produce a magnetizing field that hasno component of a fundamental phase current frequency and that interactswith an air-gap-flux produced by at least one harmonic of thefundamental current phase frequency applied to the primary side (1),said harmonic being thereby used as the base frequency for driving themotor.
 2. A linear traveling field synchronous AC electric motoraccording to claim 1, characterized in that the windings of thesuccession of toothed-modules (4) are connected to motor phase currents(5) P1 to Pn so as to display the following electrical pattern: P1 −P1,−P2 P2, P3 −P3 . . . Pn −Pn, −P1 P1, P2 −P2, −P3 P3, . . . −Pn Pn andany electromagnetically invariant equivalents thereof.
 3. A lineartraveling field synchronous AC electric motor according to claim 1,characterized in that the ratio between the motor secondary side polelength (6) and primary side slot spacing is τ_(p)/τ_(s)=6/5.
 4. A lineartraveling field synchronous AC electric motor according to claim 1,characterized in that the ratio between the motor secondary side polelength (6) and primary side slot spacing is τ_(p)/τ_(s)=6/7.
 5. A lineartraveling field synchronous AC electric motor according to claim 1,characterized in that the ratio between the motor secondary side polelength (6) and primary side slot spacing is τ_(p)/τ_(s)=6/11.
 6. Alinear traveling field synchronous AC electric motor according to claim1, characterized in that the ratio between the motor secondary side polelength (6) and primary side slot spacing is τ_(p)/τ_(s)=6/13.
 7. Alinear traveling field synchronous AC electric motor according to claim1, characterized in that the ratio between the motor secondary side polelength (6) and primary side slot spacing is higher than 6/13.
 8. Alinear traveling field synchronous AC electric motor with concentratedwindings (2) according to claim 1, characterized in that the ratio ofsecondary side pole length τp (6) to stator slot spacing τm (7) is suchthat${\frac{\tau\; p}{\tau\; n} = {\frac{m^{\prime}}{m^{\prime} \pm 1} \cdot q^{\prime}}},$whereby for a symmetrical winding the number of virtual motor phases m′is equal to the number of real motor phases (5) m, and for anasymmetrical winding is equal to 2×m, and whereby the factor q′ is equalto half the length of one toothed-module (4), i.e${q^{\prime} = \frac{z}{2}},$ when the module is wound with a free slot(13), and otherwise ${q^{\prime} = {q = \frac{N}{2 \cdot p \cdot m}}},$a whole number, where p represents the number of motor primary sidepoles (3), and N represents the number of primary side slots (13).
 9. Alinear traveling field synchronous AC electric motor with concentratedwindings (2) according to claim 1, characterized in that the pole teeth(9) have practically parallel sides (8) resulting in a tooth structurewith no pole shoe.
 10. A linear traveling field synchronous AC electricmotor with concentrated windings (2) according to claim 1, characterizedin that the motor primary side (1) is constructed from laminations whichare themselves formed as a single piece.
 11. A linear traveling fieldsynchronous AC electric motor with concentrated windings (2) accordingto claim 1, characterized in that the primary side windings (2) arepre-wound, inserted and fixed into the already completed statorstructure (1).
 12. A linear traveling field synchronous AC electricmotor with concentrated windings (2) according to claim 1, characterizedin that the number of electrical phases (5) ‘P’ is equal to three.
 13. Alinear traveling field synchronous linear AC electric motor withconcentrated windings (2) and more than one primary side pole pair (3)according to claim 1, characterized in that one sequence oftoothed-modules (4), physically at one extremity of the motor primaryside, are concentrated to motor phases (P1 to Pn so as to display apartial sequence P1 −P1, −P2 P2, P3 −P3, . . . Pn −Pn or anyelectromagnetic equivalent thereof.